In a nonvolatile semiconductor memory device, for instance, a tunnel insulating film is provided on a semiconductor substrate. A floating gate layer is provided on the tunnel insulating film. A block insulating film is provided on the floating gate layer. A gate electrode is provided on the block insulating film.
From the viewpoint of writing characteristics, it is desirable to achieve a high electron injection efficiency of injecting electrons from the semiconductor substrate through the tunnel insulating film into the floating gate layer. On the other hand, from the viewpoint of charge retention characteristics, it is desirable that the electrons accumulated in the floating gate layer be prevented from flowing through the tunnel insulating film to the semiconductor substrate as much as possible. There is demand for a nonvolatile semiconductor memory device having both of these characteristics.